U.S. patent application number 12/868209 was filed with the patent office on 2011-03-17 for fixing device and image forming apparatus using same.
Invention is credited to Seiji SAITOH.
Application Number | 20110064440 12/868209 |
Document ID | / |
Family ID | 43730666 |
Filed Date | 2011-03-17 |
United States Patent
Application |
20110064440 |
Kind Code |
A1 |
SAITOH; Seiji |
March 17, 2011 |
FIXING DEVICE AND IMAGE FORMING APPARATUS USING SAME
Abstract
The fixing device includes first and second heat sources, a
rotatable fixing member heated by the first heat source, a
rotatable pressing member heated by the second heat source and
contacting the fixing member to form a fixing nip therebetween at
which a toner image on a sheet is fixed thereto, and at least first
and second temperature detectors determining temperatures of
different surface portions of the pressing member. The pressing
member has a width greater than that of the fixing member greater
than the maximum width of the sheet accommodated by the fixing
device. The first temperature detector determines the temperature
of a surface portion of the pressing member, which is to be
contacted with the sheet, and the second temperature detector
determines the temperature of a second surface portion of the
pressing member, which is not contacted with the fixing member even
when the pressing member is rotated.
Inventors: |
SAITOH; Seiji; (Zama-shi,
JP) |
Family ID: |
43730666 |
Appl. No.: |
12/868209 |
Filed: |
August 25, 2010 |
Current U.S.
Class: |
399/69 |
Current CPC
Class: |
G03G 15/2042
20130101 |
Class at
Publication: |
399/69 |
International
Class: |
G03G 15/20 20060101
G03G015/20 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 14, 2009 |
JP |
2009-211618 |
May 19, 2010 |
JP |
2010-115405 |
Claims
1. A fixing device for fixing a toner image on a recording sheet,
comprising: a first heat source; a second heat source; a rotatable
fixing member heated by the first heat source and having a width
greater than a maximum width of the recording sheet accommodated by
the fixing device; a rotatable pressing member heated by the second
heat source and contacting the fixing member to form a fixing nip
therebetween at which a toner image on a recording sheet passing
therethrough is fixed thereto, the pressing member having a width
greater than the width of the fixing member; and at least first and
second temperature detectors configured to determine temperatures
of different surface portions of the pressing member, the first
temperature detector determining a temperature of a first surface
portion of the pressing member, which is contacted with the
recording sheet when the recording sheet is fed to the fixing nip,
and the second temperature detector determining a temperature of a
second surface portion of the pressing member, which is not
contacted with the fixing member even when the pressing member is
rotated.
2. The fixing device according to claim 1, further comprising: a
controller configured to selectively control the second heat source
based on the temperature detected by the first temperature detector
or the temperature detected by the second temperature detector.
3. The fixing device according to claim 2, wherein when the
controller performs heat control in a start-up operation time of
the fixing device, the controller selects either the temperature
detected by the first temperature detector or the temperature
detected by second temperature detector according to preset
information.
4. The fixing device according to claim 2, wherein in a returning
time of the fixing device following a waiting time thereof, the
controller performs heat control based on the temperature detected
by the second temperature detector.
5. The fixing device according to claim 2, wherein in a continuous
sheet feeding time of the fixing device, the controller switches
from heat control based on the temperature detected by the second
temperature detector to heat control based on the temperature
detected by the first temperature detector.
6. The fixing device according to claim 2, wherein when one of the
first and second temperature detectors detects an abnormal
temperature, the controller performs heat control based on the
temperature detected by the other temperature detector.
7. An image forming apparatus comprising: an image forming device
configured to form a toner image on a recording sheet; and the
fixing device according to claim 1 configured to fix the toner
image to the recording sheet.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a fixing device for fixing
a toner image to a receiving material upon application of heat and
pressure thereto, and to an image forming apparatus having the
fixing device.
[0003] 2. Discussion of the Related Art
[0004] Electrophotographic image forming apparatuses such as
copiers and laser printers form an image by performing the
following processes:
[0005] (1) forming an electrostatic latent image on an image
bearing member;
[0006] (2) developing the electrostatic latent image using a
developing device to form an unfixed toner image on the image
bearing member;
[0007] (3) transferring the unfixed toner image onto a receiving
material (such as paper sheets); and
[0008] (4) fixing the toner image to the receiving material using a
fixing device.
[0009] A key component is the fixing device, and various fixing
devices have been proposed. Specific examples thereof include heat
roller fixing devices and belt fixing devices. In heat roller
fixing devices, a receiving material bearing an unfixed toner image
is passed through a fixing nip formed by a fixing roller having a
heater therein and a pressing roller contacting the fixing roller,
so that the unfixed toner image is fixed to the receiving material
upon application of heat and pressure at the fixing nip. In belt
fixing devices, a receiving material bearing an unfixed toner image
is passed through a fixing nip formed by an endless fixing belt
tightly stretched by a fixing roller and a heating roller while
heated by the heating roller and pressed against a pressing roller
contacting the fixing roller with the fixing belt therebetween, so
that the unfixed toner image is fixed to the receiving material
upon application of heat and pressure at the fixing nip.
[0010] Since there is an optimum temperature for fixing a toner
image, the temperature of each of the fixing member (whether a
roller or a belt) and the pressing member is typically controlled
so as to fall in a temperature range including the optimum
temperature by automatically providing or turning off electricity
to heat sources for heating the fixing member and the pressing
member. Such automatic control is achieved, for example, by a
method including measuring the temperatures of a fixing member and
a pressing member using a contact temperature detector (such as a
thermistor) contacted with the fixing member and pressing member or
a non-contact temperature detector (such as thermopiles using
infrared rays), and sending the detected temperature information to
a temperature controller to control power supply to heat sources
for heating the fixing member and the pressing member.
[0011] However, even when the temperature is controlled as
described above, the fixation property of a fixed image still
varies somewhat depending on ambient temperature and humidity.
Specifically, when the ambient temperature is low, the temperature
of a receiving material declines, thereby degrading the fixation
property of an image fixed on the receiving material. Similarly,
when the ambient humidity is high, a recording paper absorbs a
large amount of moisture, thereby degrading the fixation property
of an image fixed on the receiving material.
[0012] In attempting to prevent deterioration of the fixation
property, a fixing device is proposed which detects the ambient
temperature and humidity using a detector to control the
temperature of a fixing member based on the output from the
detector. In addition, an image forming apparatus is proposed in
which a temperature difference between a fixing member and a
pressing member is controlled so as to fall in a predetermined
range to maintain the fixation property.
[0013] However, heat transfer from the fixing member to the
pressing member is not considered in the proposed fixing device and
image forming apparatus. Therefore, the temperature of the pressing
member cannot be controlled in the desired range, resulting in
formation of a curled print.
[0014] For these reasons, the inventor recognized that there is a
need for an image forming apparatus capable of producing prints
having a good fixing property without curling.
SUMMARY
[0015] This patent specification describes a novel fixing device
for fixing a toner image formed on a recording sheet, one
embodiment of which includes a heating member heated by a first
heat source, a pressing member heated by a second heat source and
contacting the heating member to form a fixing nip therebetween at
which the toner image is fixed to the recording sheet, and at least
two temperature detectors configured to determine temperatures of
different surface portions of the pressing member. The width of the
pressing member is greater than the width of the fixing member
which is greater than the maximum width of the recording sheet. One
of the temperature detectors detects the temperature of a first
surface portion of the pressing member, which portion is contacted
with the recording sheet when the recording sheet is fed to the
fixing nip, and another temperature detector detects the
temperature of a second surface portion of the pressing member,
which portion is not contacted with the fixing member even when the
pressing member is rotated.
[0016] This patent specification further describes a novel image
forming apparatus, one embodiment of which includes an image
forming device configured to form a toner image on a recording
sheet, and the above-mentioned fixing device configured to fix the
toner image to the recording sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] A more complete appreciation of aspects of the invention and
many of the attendant advantage thereof will be readily obtained as
the same become better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings, wherein:
[0018] FIG. 1 is a schematic view illustrating a color laser
printer as one example of the image forming apparatus of the
present invention;
[0019] FIG. 2 is a schematic cross-sectional view illustrating a
roller fixing device for use in the image forming apparatus;
[0020] FIG. 3 is a graph illustrating temperature distribution of a
pressing member in the thickness direction thereof;
[0021] FIG. 4 is a schematic cross-sectional view illustrating a
pressing member for use in the fixing device of the present
invention;
[0022] FIG. 5 is a schematic view illustrating an example of the
fixing device of the present invention;
[0023] FIG. 6 is a schematic view illustrating a comparative fixing
device;
[0024] FIG. 7 is a graph illustrating temperature control of a
pressing member in a start-up operation time of the image forming
apparatus of the present invention;
[0025] FIG. 8 is a graph illustrating temperature control of a
pressing member of the fixing device of the present invention in a
period from a waiting time to a returning time;
[0026] FIG. 9 is a graph illustrating temperature distribution of a
pressing member in the width direction thereof;
[0027] FIG. 10 is a schematic view illustrating a single-belt
fixing device as one example of the fixing device of the present
invention;
[0028] FIG. 11 is a schematic view illustrating another single-belt
fixing device as one example of the fixing device of the present
invention; and
[0029] FIG. 12 is a schematic view illustrating a twin-belt fixing
device as one example of the fixing device of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] In the present application, "waiting time (or waiting
state)" of an image forming apparatus is defined as a time (state)
in which the image forming apparatus has not yet performed an image
forming operation after performing a start-up operation, and each
of a fixing member and a pressing member of a fixing device of the
apparatus maintains a predetermined temperature lower than a
fixable temperature.
[0031] In addition, "returning time (state)" is defined as a time
(state) of the image forming apparatus in which the fixing member
and pressing member in the waiting state are heated to respective
predetermined temperatures.
[0032] Further, "start-up operation time (state)" is defined as a
time (state) of the image forming apparatus in which the image
forming apparatus is started up by turning on a main switch
thereof.
[0033] Furthermore, "sheet feeding time (state)" is defined as a
time (state) of the image forming apparatus of from an order of
printing (i.e., start of an image forming operation) to discharging
of a print (i.e., detection of discharging of a print with a
discharge detector).
[0034] In addition, "continuous sheet feeding time (state)" is
defined as a time (state) in which the above-mentioned printing
operation is continuously performed to produce multiple prints.
[0035] FIG. 1 is a schematic view illustrating a color laser
printer as one example of the image forming apparatus of the
present invention.
[0036] Referring to FIG. 1, the color laser printer includes a
tandem image forming section, in which yellow, cyan, magenta and
black image forming units 101Y, 101C, 101M and 101K serving as
toner image forming devices are arranged side by side in this
order. In this regard, suffixes Y, C, M and K mean that the parts
and devices are used for forming yellow, cyan, magenta and black
color images, respectively, and the suffixes are sometimes omitted.
The image forming units 101 include respective photoreceptors 21Y,
21C, 21M and 21K serving as electrostatic latent image bearing
members. In addition, the image forming units 101 include
developing devices 10Y, 10C, 10M and 10K configured to develop
electrostatic latent images on the photoreceptors 21 with
respective color developers to form yellow, cyan, magenta and black
color toner images on the photoreceptors, cleaners 14 configured to
clean the surfaces of the respective photoreceptors, chargers 13
configured to charge the respective photoreceptors, etc., which
devices are arranged around the photoreceptors. In addition, the
printer has four toner bottles 2Y, 2C, 2M and 2K configured to
contain respective color toners and arranged on an upper portion
thereof. The color toners in the toner containers 2 are supplied in
proper amounts to the respective developing devices 10Y, 10C, 10M
and 10K through respective feeding passages so that the developers
in the developing devices maintain predetermined toner
concentrations.
[0037] A light irradiating device 9 serving as an electrostatic
latent image forming device is provided below the tandem image
forming section. The light irradiating device 9 includes a light
source, a polygon mirror, a f-.theta. lens, a reflection mirror,
etc., and scans each photoreceptor 21 with laser light according to
image data to form an electrostatic latent image on the
photoreceptor.
[0038] An endless intermediate transfer belt 1 serving as an
intermediate transfer medium is provided above the tandem image
forming section. The intermediate transfer belt 1 is tightly
stretched by a driving roller 1a and a driven roller 1b, wherein a
driving motor (not shown) is connected with a shaft of the driving
roller. When the driving motor is driven, the driving roller 1a is
rotated, and the intermediate transfer belt 1 is rotated
(counterclockwise in FIG. 1) while the driven roller 1b is rotated
by the intermediate transfer belt. Primary transfer members 11Y,
11C, 11M and 11K are provided inside the intermediate transfer belt
1 to sequentially transfer color toner images formed on the
photoreceptors 21Y, 21C, 21M and 21K onto the intermediate transfer
belt, resulting in formation of a combined color toner image on the
intermediate transfer belt.
[0039] A secondary transfer roller 4 serving as a secondary
transfer member is provided downstream from the primary transfer
members 11 relative to the moving direction of the intermediate
transfer belt 1. The secondary transfer roller 4 is opposed to the
driven roller 1b with the intermediate transfer belt 1 therebetween
to press the intermediate transfer belt to the driven roller,
thereby forming a secondary transfer nip.
[0040] The printer further includes a recording sheet cassette 8
configured to contain a stack of recording sheets S, a sheet
feeding roller 7 configured to feed the uppermost recording sheet,
a pair of registration rollers 6, etc.
[0041] A fixing device 5 configured to fix a toner image on the
recording sheet S, and a pair of discharging rollers 3 are provided
downstream from the secondary transfer roller 4 relative to the
feeding direction of the recording sheet S.
[0042] Next, the operation of the printer will be described.
[0043] In each image forming unit 101, the photoreceptor 21 is
rotated, and the charger 13 evenly charges the surface of the
photoreceptor. Next, the light irradiating device 9 irradiates the
charged photoreceptor 21 with laser light to form an electrostatic
latent image on the photoreceptor. The developing device 10
develops the electrostatic latent image with a color toner to form
a color toner image on the photoreceptor 21. Thus, yellow, cyan,
magenta and black color images are formed on the photoreceptors
21Y, 21C, 21M and 21K, respectively. In addition, a driving motor
(not shown) rotates the driving roller 1a to rotate the
intermediate transfer belt 1 while rotating the driven roller 1b
and the second transfer roller 4. The color toner images formed on
the photoreceptors 21Y, 21C, 21M and 21K are sequentially
transferred onto the intermediate transfer belt 1 by the respective
primary transfer members 11Y, 11C, 11M and 11K, resulting in
formation of a combined color toner image on the intermediate
transfer belt. After the color toner images are transferred, the
surfaces of the photoreceptors 21 are cleaned by the respective
cleaners 14 so as to be ready for the next image forming
operation.
[0044] Meanwhile, the recording sheet S is fed from the sheet
cassette 8 by the feeding roller 7. The thus-fed recording sheet S
is stopped once by the pair of registration rollers 6, and then fed
timely toward the secondary transfer nip formed by the secondary
transfer roller 4 and the intermediate transfer belt 1 so that the
combined toner image on the intermediate transfer belt is
transferred onto a proper position of the recording sheet at the
secondary transfer nip.
[0045] After the secondary transfer operation, the recording sheet
S bearing the combined color toner image thereon is fed to the
fixing device 5 so that the combined color toner image is fixed to
the recording sheet upon application of heat and pressure at a
fixing nip formed by a fixing roller 51 having a first heat source
53 therein and a pressing roller 52 having a second heat source 54
therein and contacting the fixing roller 51. The recording sheet S
bearing the fixed combined color toner image (i.e., a full color
image) thereon is then discharged from the main body of the printer
by the pair of discharging rollers 3, resulting in formation of a
full color print. After the secondary transfer operation, the
surface of the intermediate transfer belt 1 is cleaned by a belt
cleaner 12 so that the intermediate transfer belt is ready for the
next primary transfer operation.
[0046] Next, the configuration of the fixing device of the present
invention will be described. Hereinafter, a roller fixing device
will be described as one example of the fixing device of the
present invention, but the present invention is not limited
thereto.
[0047] The fixing device 5 illustrated in FIG. 2, which is an
example of the fixing device of the present invention, includes the
fixing roller 51 serving as a fixing member, the pressing roller 52
serving as a pressing member, the first heat source 53 for heating
the fixing roller, the second heat source 54 for heating the
pressing roller, temperature detectors 55 and 57, a separating
member 58 for separating the recording sheet S from the fixing
roller, a guide plate (not shown), etc. Although in FIG. 2 the
temperature detectors 55 and 57 are respectively contacted with the
pressing roller 52 and the fixing roller 51, the detectors need not
necessarily be contacted therewith provided they are able to detect
the temperatures of the pressing roller 52 and the fixing roller
51.
[0048] The fixing roller 51 has such a configuration that an
elastic layer made of a foamed material such as foamed silicone
rubbers is formed on a metal shaft, so that a desired fixing nip
can be formed between the fixing roller 51 and the pressing roller
52.
[0049] The first and second heat sources 53 and 54 are respectively
provided inside the fixing roller 51 and pressing roller 52. A
controller (illustrated in FIG. 5) controls supply of electricity
to the first and second heat sources based on the temperatures
detected by the temperature detectors 55 and 57 such as thermistors
to control the temperatures of surfaces of the fixing roller 51 and
pressing roller 52. Specific examples of the first and second heat
sources include halogen heater, infrared heaters, induction
heaters, thermal resistors, etc.
[0050] The fixing device 5 operates such that when a main switch of
the printer is turned on, a high frequency power is supplied to the
first and second heat sources 53 and 54 by an electric power
source, and the fixing roller 51 and pressing roller 52 start to be
rotated.
[0051] The recording sheet S bearing the combined color toner image
thereon is fed to the fixing nip of the fixing device 5 as
indicated by an arrow illustrated in FIG. 2. The combined color
toner image is fixed to the recording sheet S at the fixing nip by
the heat and pressure applied by the fixing roller 51 and pressing
roller 52. After passing the fixing nip, the recording sheet S is
separated from the fixing roller 51 by the separating member
58.
[0052] Next, the effect of heat of a fixing member (fixing roller
51) on the temperature of the surface of a pressing member
(pressing roller 52) will be described with reference to FIG. 3,
which illustrates temperature distribution of the pressing member
in the thickness direction thereof.
[0053] In this regard, image forming apparatuses, in operation,
typically set the temperature of a fixing member thereof so as not
to be lower than the temperature of a pressing member thereof. In
addition, the heat capacity of the fixing member is typically
smaller than that of the pressing member, and the power of a heat
source for heating the fixing member is higher than the power of a
heat source for heating the pressing member. Therefore, the fixing
member thereof typically has much better thermal responsiveness
than the pressing member thereof.
[0054] In FIG. 3, the "inner surface" means the inner surface of a
pressing member, for example, the inner surface of a substrate 60
of the pressing roller 52 illustrated in FIG. 4, and the "outer
surface" means the outer surface of the pressing member, for
example, the outer surface of a rubber layer 59 of the pressing
roller 52 illustrated in FIG. 4. Therefore, the thickness of the
pressing member, i.e., the distance between the inner surface and
the outer surface thereof, is plotted on the horizontal axis of
FIG. 3. Specifically, in FIG. 3, the temperature distribution of
from the inner surface to the outer surface of the pressing roller
in a waiting state is represented with a chain line, the
temperature distribution in a state just after start of rotation is
represented with a broken line, and the temperature distribution in
a sheet feeding state is represented with a solid line.
[0055] In FIG. 3, a temperature (T1) of the outer surface of the
pressing member in the waiting time is the same as that in the
sheet feeding time. The outer surface of the pressing member must
have the temperature T1 in the sheet feeding time. Therefore, the
pressing member maintains the temperature T1 even in the waiting
time so as to be ready for a sheet to be fed to the fixing nip. In
order that the outer surface of the pressing member maintains the
temperature T1, the inner surface thereof has to have a temperature
T2 represented by a black diamond mark (.diamond-solid.) in the
waiting time.
[0056] In the sheet feeding time, the sheet passing through the
fixing nip draws heat from the pressing member. Therefore, in order
that the pressing member maintain the temperature T1, it is
necessary to apply heat to the pressing member in a quantity
greater than that in the waiting time. Therefore, the inner surface
of the pressing member has to maintain a predetermined temperature
T3 that is higher than the temperature T2. Since the quantity of
heat transferred from the inner surface to the outer surface (i.e.,
from the inner surface to the sheet) is larger than the quantity of
heat transferred in the waiting time, the pressing member has the
temperature distribution indicated by the solid line in FIG. 3.
Specifically, in the sheet feeding time, heat is continuously
applied from the inner surface to the outer surface to compensate
for the heat drawn by the sheet, i.e., the pressing member in the
sheet feeding state has a larger quantity of heat than in the
waiting time. The difference in heat quantity corresponds to an
area (deficiency of heat quantity) illustrated by slanted lines in
FIG. 3. The deficiency of heat quantity will be described in
greater detail later.
[0057] The temperature distribution of the pressing member will now
be described in detail. Since the fixing roller 51 is not rotated
in the waiting time, heat transfer is performed only through the
fixing nip, and the quantity of heat transferred from the fixing
roller to the pressing roller is small. Since the temperature
detector 55 for determining the temperature of the surface of the
pressing member 52 cannot be disposed at the nip of the pressing
member contacted with the fixing member 51 but must be disposed on
a portion other than the nip, the heat of the fixing roller hardly
influences the temperature of the pressing roller detected by the
temperature detector 56.
[0058] By contrast, in the state in which the fixing roller 51
rotates, eventually, the entire surface of the fixing roller 51 is
contacted with the pressing roller 52 through the nip. In this
case, since the temperature of the fixing roller 51 is higher than
that of the pressing roller 52, heat is transferred from the fixing
roller to the pressing roller.
[0059] As a result, in the time just after start of rotation of the
fixing roller 51 illustrated by the broken line in FIG. 3, i.e., a
rotation starting time of from start of rotation of the fixing
roller to arrival of the sheet to the fixing nip, the temperature
of the pressing roller 52 increases as illustrated by a black
circle mark ( ). However, only the temperature of the outer surface
of the pressing roller 52 is increased to a temperature T4 in the
rotation starting time, and the inner surface of the pressing
roller hardly increases because the heat of the fixing roller
hardly reaches the inner surface of the pressing roller. Namely,
the temperature of the pressing roller 52 is substantially the same
as that in the waiting time except for the outer surface thereof.
In other words, the pressing roller 52 experiences an apparent
temperature increase, and has the deficiency of heat quantity as
mentioned above.
[0060] In this regard, the temperature of the surface of the
pressing roller 52 is detected by the temperature detector 55, and
if the surface temperature decreases due to start of rotation of
the fixing roller 51, the heat source 54 supplies heat to the
pressing roller from inside. However, as stated, in the
above-mentioned rotation starting state, the temperature of the
outer surface of the pressing roller 52 is increased by the heat
from the fixing roller 51, even though the inner portion of the
pressing roller is not sufficiently heated (i.e., the whole of the
pressing roller 52 is not yet heated). Therefore, the image forming
apparatus judges that it is not necessary to heat the pressing
roller, and stops supplying heat to the pressing roller.
[0061] At this point, when the recording sheet S passes through the
fixing nip under such conditions, the sheet draws heat from the
pressing roller 52, thereby rapidly decreasing the temperature of
the surface of the pressing roller 52, resulting in a net
deficiency of heat quantity of the pressing roller. Even though the
heat quantity deficiency can be detected and heat is supplied to
the pressing roller 52, the temperature of the pressing roller 52
cannot be rapidly raised to the predetermined temperature (i.e., it
takes a relatively long time until the pressing roller 52 is heated
to the predetermined temperature) because the pressing roller has a
considerable thickness and heat capacity, and the power supplied to
the heat source 54 is limited. Since the temperature of the surface
of the pressing roller 52 is thus affected by the fixing roller 51,
the temperature control of the pressing roller cannot be
satisfactorily performed, resulting in occurrence of the
above-mentioned curling problem, thereby degrading the feeding
property of the recording sheet and the fixing property, such that
the toner image cannot be satisfactorily fixed to the recording
sheet.
[0062] FIG. 4 is a schematic cross-sectional view illustrating the
pressing roller 52 for use as the pressing member of the fixing
device of the present invention. Referring to FIG. 4, the pressing
roller 52 has the rubber layer 59 serving as an elastic layer and
made of a rubber such as silicone rubbers so that a fixing nip
having a predetermined length can be formed between the pressing
roller and the fixing roller 51. In addition, the pressing roller
52 has the cylindrical substrate 60, which is made of a metal (such
as iron and iron alloys) and has a considerable thickness so as to
maintain the shape thereof even when receiving a pressing force
from the fixing roller 52. For example, the thicknesses of the
rubber layer 59 and substrate 60 are preferably 3 mm and 1 mm,
respectively.
[0063] Therefore, the pressing roller 52 has a relatively large
heat capacity. In addition, the fixing property of a toner image is
largely influenced by the temperature of the fixing roller 51.
Specifically, preferably the temperature of the fixing roller 51 is
controlled primarily and raised more quickly than that of the
pressing roller 52. Therefore, it is preferable that the fixing
roller 51 have a small heat capacity, and more power is applied to
the heat source 53 for heating the fixing roller than that applied
to the heat source 54 for heating the pressing roller 52. In this
regard, the power consumption of the printer has an upper limit and
is as low as possible in view of the need for energy efficiency.
Therefore, it is typical that the heat source 54 cannot supply a
sufficient quantity of heat to the pressing roller 52, resulting in
deterioration of the thermal responsiveness of the pressing
roller.
[0064] In order to solve these problems, the fixing device 5, which
is one example of the fixing device of the present invention,
includes the fixing roller 51, the pressing roller 52 contacting
the fixing roller to form a fixing nip at which an unfixed toner
image on the recording sheet S is fixed thereto, the first heat
source 53 configured to heat the fixing roller 51, the second heat
source 54 configured to heat the pressing roller 52, and at least
two temperature detectors configured to determine the temperatures
of different surface portions of the pressing roller. The width of
the pressing roller 52 (i.e., the length of the pressing roller in
the longitudinal, or axial, direction thereof) is greater than that
of the fixing roller 51 as illustrated in FIG. 5. As illustrated in
FIG. 5, the first temperature detector 55, one of the temperature
detectors, is provided to measure the temperature of a central
surface portion of the pressing roller 52 within a sheet feeding
area, and a second temperature detector 56 is provided to measure
the temperature of an end surface portion of the pressing roller,
which portion is not contacted with the fixing roller even when the
fixing roller and pressing roller rotate. Although only one
temperature detector is used for each of the temperature detectors
55 and 56 in FIG. 5, plural temperature detectors can be used
therefor as illustrated in FIG. 9.
[0065] If the width of the pressing roller 52 is less than that of
the fixing roller 51 as illustrated in FIG. 6 and a temperature
detector 55a is arranged to measure the temperature of a surface of
a portion of the pressing roller without the sheet feeding area,
the temperature of the portion is affected by the fixing
roller.
[0066] For the reason mentioned above, in the fixing device of the
present invention the pressing roller 52 is wider than the fixing
roller 51, and the second temperature detector 56 is arranged to
measure the temperature of an end surface portion of the pressing
roller, which portion is not contacted with the fixing roller,
i.e., which does not form a fixing nip with the fixing roller as
illustrated in FIG. 5.
[0067] The fixing device 5 having such a configuration can control
the temperature of the pressing roller 52 without being affected by
the heat of the fixing roller.
[0068] Hereinafter, an example of the temperature control of the
pressing roller 52 according to the present invention will be
described, but the temperature controlling is not limited
thereto.
[0069] The fixing device of the present invention preferably has a
controller 70 (illustrated in FIG. 5) configured to control the
heat source 54 according to the detection signals from the first
and second temperature detectors 55 and 56. The controller 70
properly selects the first temperature detector or second
temperature detector depending on the state of the fixing device 5,
and controls the heat source 54 based on the detection signal of
the selected temperature detector to control the temperature of the
pressing roller 52.
[0070] At first, temperature control of the pressing roller 52 in
the start-up operation time of the printer will be described with
reference to FIG. 7.
[0071] In the start-up operation time of the printer, power is
supplied to each of the heat source 53 for heating the fixing
roller 51 and the heat source 54 for heating the pressing roller
52, and the fixing roller and pressing roller are rotated so that
the entire surface of each roller has almost the same
temperature.
[0072] In FIG. 7, a curve A represents temporal change of the
temperature of a surface of the pressing roller 52 in the start-up
operation time detected by the second temperature detector 56, a
curve B represents temporal change of the temperature of a surface
of the pressing roller 52 in the start-up operation time detected
by the first temperature detector 56, and a curve C represents
temporal change of the temperature of a surface of the fixing
roller 51 in the start-up operation time detected by the fixing
roller temperature detector 57.
[0073] The present inventor confirmed that when the pressing roller
52 is heated only by the heat source 54, the temperature of the
surface of the pressing roller 52 changes like the curve A.
Therefore, the curve A accurately represents the actual temperature
of the pressing roller 52. Accordingly, it is preferable that a
fixing operation is performed only after the temperature A reaches
the target temperature of the pressing roller. By contrast, the
temperature B of a surface of the pressing roller 52 detected by
the first temperature detector 56 increases more quickly than the
temperature A because the surface of the pressing roller receives
is heated by the fixing roller 51. If the controller 70 operates
based on the temperature detected only by the first temperature
detector 55, the controller judges that the pressing roller 52 is
heated to the target temperature (i.e., the start-up operation is
completed) and stops rotation of the rollers prematurely, even
though the whole of the pressing roller is not sufficiently
heated.
[0074] When the rotation of the fixing roller 51 and pressing
roller 52 are stopped, heat is not supplied from the fixing roller
to the pressing roller, thereby decreasing the temperature of the
surface of the pressing roller 52 as illustrated by the broken line
in FIG. 7. Finally the temperature B becomes substantially the same
as the temperature A. If the printer performs an image forming
operation based on this determination of completion of the start-up
operation, however, the recording sheet S passing through the
fixing nip draws heat from the pressing roller 52, resulting in
decrease of the temperature of the pressing roller, thereby largely
curling the recording sheet (i.e., forming a curled print).
[0075] Therefore, in the fixing device of the present invention,
the controller operates based on the temperature detected by the
second temperature detector 56 to reduce the degree of curling of
the recording sheet S after the fixing operation, resulting in
prevention of occurrence of a jamming problem in that the recording
sheet jams in a passage of from the fixing device 5 to the pair of
discharging rollers 3 (i.e., resulting in improvement of feeding
property of the recording sheet), and a defective stacking problem
in that prints are defectively stacked on a copy tray due to
curling thereof.
[0076] However, when performing temperature control based on the
temperature detection signal from the second temperature detector
56 to reduce the degree of curling of the recording sheet 5, the
start-up operation takes a relatively long time. Therefore, the
printer has two settable conditions, enabling a determination to be
made whether to place a higher priority on reduction of curling or
shortening of the start-up operation time By selecting temperature
detection by the first temperature detector 55 or temperature
detection by the second temperature detector 56 as appropriate, it
becomes possible to reduce curling of the recording sheet or
shorten the start-up operation time according to demand of the
user.
[0077] It is preferable to previously determine a proper rotation
stopping time empirically based on the detection signals of both
the first and second temperature detectors 55 and 56. For example,
it is preferable to perform controlling such that at a time in a
period D illustrated in FIG. 7, which is an empirically
predetermined time after the temperature B reaches the target
temperature, rotation of the rollers is stopped or the image
forming operation is started.
[0078] Next, temperature control of the pressing roller in the
sheet feeding time of the printer following the waiting time
thereof will be described. When temperature control is performed
based on the temperature detected by the first temperature detector
55 illustrated by a broken line in FIG. 8, the temperature of the
surface of the pressing roller 52 shows an apparent increase (as
illustrated by the broken line in FIG. 3) when the fixing roller 51
and pressing roller start to rotate, due to the heat transferred
from the fixing roller. Therefore, the controller 70 stops supply
of power to the heat source 54 in a period E illustrated in FIG. 8.
When the recording sheet S is fed to the fixing nip under such a
condition, the recording sheet draws heat from the surface of the
pressing roller 52, thereby rapidly decreasing the temperature of
the pressing roller. In this regard, when the first temperature
detector 55 detects that the temperature of the pressing roller 52
is lower than the target temperature, power supply to the heat
source 54 is restarted. However, since the inner portion of the
pressing roller 52 is lacking in heat quantity in the
non-activating period (i.e., the period E) due to stopping of the
supply of power to the heat source 54 based on detection of the
apparent temperature of the pressing roller, it takes time for the
heat to be transferred to the surface of the pressing roller, and
the temperature of the surface of the pressing roller falls sharply
so as to be lower by G than the target temperature as illustrated
in FIG. 8.
[0079] By contrast, when heat controlling is performed based on the
temperature detected by the second temperature detector 56 as
illustrated by a solid line in FIG. 8, the temperature of the end
portion of the pressing roller 52 is controlled without being
affected by the heat of the fixing roller 51 because heat is not
transferred to the end portion even when the rollers rotate.
Therefore, the pressing roller 51 maintains the proper state even
when the rollers rotate. When the recording sheet S is fed to the
fixing nip under such conditions, the temperature of the pressing
roller 52 changes like a curve illustrated by the solid line in
FIG. 8 while the temperature drop (i.e., B in FIG. 8) of the
pressing roller is minimized. This is because the pressing roller
52 has been subjected to proper heat control until just before
contact of the roller with the recording sheets, and power starts
to be supplied to the heat source 54. Therefore, the temperature of
the pressing roller 52 can be rapidly returned to the target
temperature.
[0080] As mentioned above, in the sheet feeding state of the
printer following the waiting state thereof, the controller 70
controls the temperature of the pressing roller 52 based on the
temperature detected by the second temperature detector 56, and
therefore no image forming operation is performed when the
temperature of the pressing roller 52 is lower than the target
temperature, thereby making it possible to reliably produce a print
having little curl and good feeding property.
[0081] FIG. 9 is a graph illustrating temperatures of seven
different points (a)-(g) along the surface of the pressing member
in the width direction thereof in the waiting time, rotation
starting time, and sheet feeding time. Among the seven points,
three points (c), (d) and (e) are located in the sheet feeding area
(i.e., the central area), two points (b) and (f) are located within
a fixing member contacting area and out of the sheet feeding area,
and other two points (a) and (g) are located out of the fixation
area.
[0082] As illustrated in FIG. 9, the temperatures of the seven
points are the same in the waiting time. However, when the fixing
member and pressing member start to rotate, the temperatures of the
five points (b)-(f) within the fixing member contacting area are
increased. By contrast, the temperatures (a) and (g) of the end
portions are not increased and maintain the temperature thereof in
the waiting time because the end portions are not affected by the
heat of the fixing member. Therefore, when temperature detection is
performed at the end points (a) and (g) out of the fixing member
contacting area, more accurate temperature detection can be carried
out unaffected by the above-mentioned apparent increase in
temperature of the pressing member. When the recording sheet is
continuously fed to the fixing device while the pressing member is
heated, the temperatures of the points (a), (b), (f) and (g) become
higher than the central points (c), (d) and (e) located in the
sheet feeding area because the recording sheet draws heat from the
central portions of the pressing member. In this regard, the points
(b) and (f) have a higher temperature than the points (a) and (g)
because of receiving heat from the fixing roller.
[0083] When control based on the temperature detected by the second
temperature detector 56 is continued even in the sheet feeding
time, the temperature of the pressing member in the sheet feeding
area gradually decreases. Therefore, it is preferable to control
based on the temperature detected by the first temperature detector
55 in the continuous sheet feeding time. Whether or not to perform
the switching operation is determined, for example, based on the
number of the continuously fed recording sheets or the continuous
sheet feeding time (i.e., continuous image forming time). Namely,
the switching operation is performed when the number of the
continuously fed recording sheets or the continuous sheet feeding
time exceeds a predetermined threshold value. It is preferable that
the user can determine the threshold value.
[0084] Performing such temperature control makes it possible to
reliably produce prints having a good fixing property while
reducing curling of the prints even at the beginning of an image
forming operation or in a continuous image forming operation.
[0085] In addition, the fixing device of the present invention can
detect abnormal temperature of the pressing member (i.e.,
abnormality of the fixing device) based on both the temperatures
detected by the first and second temperature detectors 55 and 56
more easily than a fixing device using only one temperature
detector, providing improved reliability. For example, even when
one of the temperature detectors is damaged, abnormality of the
fixing device (such as overheating of the pressing member) can be
detected by another temperature detector, and thereby controlling
such that the fixing device is stopped can be performed, resulting
in prevention of formation of defective prints and damage to the
fixing device.
[0086] The image forming apparatus of the present invention
includes an image forming device configured to form a toner image
on a recording sheet, and the above-mentioned fixing device
configured to fix the toner image to the recording sheet.
Therefore, the image forming apparatus of the present invention can
stably produce prints having a good fixing property and little curl
without causing sheet jamming and defective stacking. In this
regard, the image forming units 101, the transferring device
including the intermediate transfer belt 1, primary transfer
members 11, and secondary transfer member 4, the light irradiating
device 9 illustrated in FIG. 1, etc. constitute the image forming
device.
[0087] The fixing device of the present invention is not limited to
the above-mentioned example. That is, the above-mentioned fixing
device is a roller fixing device. However, the present invention is
not limited thereto, and can be applied to belt fixing devices as
described below.
[0088] FIG. 10 illustrates a belt fixing device having a fixing
belt 61 serving as a fixing member, a support roller 62 supporting
the fixing belt together with the fixing roller 51, and the
pressing roller 52. The temperature detectors 55 and 56 are
disposed to determine the temperatures of surface portions of the
pressing roller 52, although the temperature detector 56 is not
illustrated in FIG. 10.
[0089] FIG. 11 illustrates another belt fixing device having a
pressing belt 63, which is rotated and supported by the pressing
roller 52 and support rollers 64. The recording sheet S bearing an
unfixed toner image thereof is fed to the fixing nip formed by the
fixing roller 51 and the pressing belt 63 so that the toner image
is fixed to the recording sheet at the fixing nip. In FIG. 11,
heating sources for heating the pressing belt 63 and fixing roller
51, and temperature detectors for detecting the temperatures of the
fixing roller 51 and pressing belt, not illustrated.
[0090] FIG. 12 illustrates a twin belt fixing device having the
fixing belt 61, which is rotated and supported by the fixing roller
51 and support roller 62, and the pressing belt 63, which is
rotated and supported by the pressing roller 52 and support roller
64. In FIG. 12, heating sources for heating the fixing belt 61,
pressing belt 63, fixing roller 51, and pressing roller 52, and
temperature detectors for detecting the temperatures of the fixing
belt and pressing belt, are not illustrated.
[0091] Additional modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims the invention may be practiced other than as specifically
described herein.
[0092] This document claims priority and contains subject matter
related to Japanese Patent Applications Nos. 2009-211618 and
2010-115405, filed on Sep. 14, 2009, and May 19, 2010,
respectively, the entire contents of which are herein incorporated
by reference.
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